XMPP is based on Xml and is using UTF-8 encoded Xml only.
Depending on the Unicode character it gets represented with 1,2,3 or 4 bytes in UTF-8. This is a requirement and there is no way to change it.

If there is a bug in the server you have to wait until this bug is fixed, use another server software or exclude the problematic range of Unicode characters. Excluding characters make no sense to me, because XMPP and UTF-8 is designed to be Unicode compatible.